Screw-type extrusion machines are usually heated with heating plates or heating shells, which are arranged on the outer surfaces of the housing of the screw-type extrusion machine and which are provided in their interior with electrical resistance heating elements, which have a constant electrical resistance along their length, the heating elements releasing along their length with a corresponding voltage respectively with a corresponding current a constant specific heating capacity, i.e. a constant heating capacity per surface unit or--related to the circumference of the housing--per circumferential section. Because of the ways, which are different for constructive reasons, and which the heat current must cover from the outer surfaces of the housing to the wall of the bore resp. bores, different temperatures occur at the wall of the bore. The heat transfer from the wall of the bore to the material to be treated thus is not constant along its circumference and possibly along its length.
From the German Utility Model 68 00 967 it is known for a screw-type extrusion machine with an annular cylindrical housing, to provide in the housing ducts for a coolant on the one hand and heating elements on the other hand which are parallel to the axis and concentrically to the axis of the bore. In this special embodiment the temperature is constant at the wall of the bore along its circumference; therefore the above described problem does not occur in that case. Comparable embodiments are known from the Published German Patent Application 24 17 067 and from the U.S. Pat. No. 2 541 201.
From DE 30 23 393 A1 a double-shaft screw-type extrusion machine is known, which has tempering ducts running parallel to the axes of the bores. In the vicinity of the bore saddles these bores are guided into the saddles, in order to attain a uniform tempering of the screw housing despite of the different distances between the 8-shaped wall of the bores and the outer surface of the wear liner.
From the German Utility Model 19 04 371 an electrically heatable press plate is known, which has a uniform temperature along its total surface, and that in particular in its margin, the heating elements being closely arranged in this marginal area.